U.S. patent number 4,497,687 [Application Number 06/563,683] was granted by the patent office on 1985-02-05 for aqueous process for etching cooper and other metals.
This patent grant is currently assigned to Psi Star, Inc.. Invention is credited to Norvell J. Nelson.
United States Patent |
4,497,687 |
Nelson |
February 5, 1985 |
Aqueous process for etching cooper and other metals
Abstract
Nitrogen dioxide process for etching copper and other metals,
using water as a catalyst/solvent. In one disclosed embodiment, a
film of water is formed on the surface of the metal, and the
water-covered metal is exposed to gaseous NO.sub.2 to dissolve the
metal. In another embodiment, the metal is exposed to an aqueous
solution of NO.sub.2 or HNO.sub.3 in water, either by immersion or
by spraying, to remove the metal.
Inventors: |
Nelson; Norvell J. (Palo Alto,
CA) |
Assignee: |
Psi Star, Inc. (Hayward,
CA)
|
Family
ID: |
27059285 |
Appl.
No.: |
06/563,683 |
Filed: |
December 20, 1983 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
517943 |
Jul 28, 1983 |
|
|
|
|
501159 |
Jun 6, 1983 |
|
|
|
|
Current U.S.
Class: |
216/13; 216/106;
216/51; 252/79.2; 252/79.4 |
Current CPC
Class: |
C23F
1/12 (20130101); H05K 3/067 (20130101); C23F
1/16 (20130101) |
Current International
Class: |
C23F
1/10 (20060101); C23F 1/12 (20060101); C23F
1/16 (20060101); H05K 3/06 (20060101); C23F
001/02 (); B44C 001/22 (); C03C 015/00 (); C03C
025/06 () |
Field of
Search: |
;156/656,659.1,664,666,901,902,640 ;252/79.2,79.4,142,146,79.1
;134/3,41 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kirk-Othmer, vol. 1, pp. 312, 313, 326 and 327, and vol. 5, pp.
145-149 (1979), Encyclopedia of Chemical Technology..
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Flehr, Hohbach, Test, Albritton
& Herbert
Parent Case Text
This is a continuation-in-part of Ser. No. 517,943, filed July 28,
1983, which is a continuation-in-part of Ser. No. 501,159, filed
June 6, 1983.
Claims
I claim:
1. In a process for etching copper, the steps of: forming a film of
water on the surface of the copper, and exposing the water-covered
copper to gaseous NO.sub.2 for a time sufficient to dissolve the
copper.
2. The process of claim 1 wherein the film of water is formed on
the copper by applying a mixture of water and an additive which
serves both as a surfactant and as an inhibitor of etching in a
direction parallel to the surface of the copper.
3. The process of claim 2 wherein the additive comprises a
polymer.
4. The process of claim 3 wherein the polymer is selected from the
group consisting of a water-soluble polyacrylamide, a
carboxymethyl-cellulose, and poly(acrylic acid).
5. The process of claim 3 wherein the polymer comprises a cationic
water-soluble polyacrylamide.
6. The process of claim 2 wherein the additive is a cationic
surfactant.
7. In a process for removing copper from a printed circuit board
having a substrate beneath the copper and etch resist material
covering a portion of the copper to be retained, contacting the
board with an aqueous solution of NO.sub.2 or HNO.sub.3, and
including a sufficient quantity of a dissolved copper salt in the
solution to prevent attack of the substrate or the etch resist
material as the copper is removed.
8. The process of claim 7 wherein the copper salt is selected from
the group consisting of Cu(NO.sub.3).sub.2, CuSO.sub.4, copper (II)
tetrafluoroborate, CuCl.sub.2 and combinations thereof.
9. catalyst/solvent for use in the etching of copper with gaseous
NO.sub.2, comprising a mixture of water and an additive which
serves as a surfactant and as an inhibitor of etching in a
direction parallel to the surface of the copper.
10. The catalyst/solvent of claim 9 wherein the additive comprises
a polymer.
11. The catalyst/solvent of claim 10 wherein the polymer is
selected from the group consisting of a water-soluble
polyacrylamide, a carboxymethyl-cellulose, and poly (acrylic
acid).
12. The catalyst/solvent of claim 10 wherein the polymer comprises
a cationic water-soluble polyacrylamide.
13. In a process for etching a metal on a substrate: exposing the
metal to an aqueous solution of NO.sub.2 or HNO.sub.3 containing a
sufficient quantity of a metal nitrate to prevent etching of the
substrate as the metal is dissolved.
14. The process of claim 13 wherein the metal is selected from the
group consisting of copper, vanadium, manganese, iron, cobalt,
nickel, palladium, and alloys thereof.
15. The process of claim 13 wherein the nitrate contained in the
solution is Cu(NO.sub.3).sub.2.
16. The process of claim 13 wherein the nitrate contained in the
solution is a nitrate of the metal being etched.
17. In a process for etching copper having a lead-tin solder mask
covering a portion thereof, the steps of: forming a film of water
on the surface of the copper by applying an aqueous solution
containing a surfactant, an additive which inhibits etching of the
copper at a direction parallel to the surface and a phosphate which
prevents etching of the solder mask, and exposing the water-covered
copper to gaseous NO.sub.2 for a time sufficient to dissolve the
copper which is not covered by the solder mask.
18. The process of claim 17 wherein the aqueous solution contains
both phosphoric acid and another phosphate.
19. In a process for removing copper from a printed circuit board
having a substrate beneath the copper and a lead-tin solder mask
covering a portion of the copper, contacting the board with an
aqueous solution of NO.sub.2 or HNO.sub.3, a sufficient quantity of
a dissolved copper salt to prevent etching of the substrate, and a
phosphate to prevent etching of the solder mask.
20. The process of claim 19 wherein the aqueous solution contains
phosphoric acid and another phosphate.
21. A catalyst/solvent for use in the etching of copper with
gaseous NO.sub.2, comprising a mixture of water, a phosphate, a
surfactant and an additive which inhibits etching in a direction
parallel to the surface of the copper.
22. The catalyst/solvent of claim 21 including phosphoric acid and
another phosphate.
23. In a process of etching a metal having a lead-tin solder mask
covering a portion thereof: exposing the metal to an aqueous
solution of NO.sub.2 or HNO.sub.3 containing a metal nitrate and a
phosphate to dissolve the exposed metal.
24. The process of claim 23 wherein the aqueous solution contains
phosphoric acid and another phosphate.
25. In a process for removing copper from a printed circuit board
having a substrate beneath the copper and etch resist material
covering a portion of the copper to be retained, contacting the
board with an aqueous solution of NO.sub.2 or HNO.sub.3, a
sufficient quantity of a dissolved copper salt to prevent attack of
the substrate or the etch resist material as the copper is removed,
and a polymer additive which prevents undercutting of the copper as
it is etched.
26. The process of claim 25 wherein the polymer is selected from
the group consisting of a water-soluble polyacrylamide, a
carboxymethyl-cellulose, and poly (acrylic acid).
27. The process of claim 25 wherein the polymer comprises a
cationic water-soluble polyacrylamide.
28. The process of claim 25 wherein the aqueous solution comprises
0.5-2.0 units by weight of 10%-100% HNO.sub.3 in water, 2.5-10
units by weight of 0.1-1.0% solution of a water-soluble acrylamide
polymer in water, and 11-44 units by weight of a 10-60% solution of
Cu(NO.sub.3).sub.2 in water.
29. In an aqueous solution for etching copper: NO.sub.2 or
HNO.sub.3, a dissolved copper salt, a surfactant and an additive to
inhibit undercutting of the copper as it is etched.
30. The solution of claim 29 wherein the surfactant and the
additive are a polymer.
31. The solution of claim 30 wherein the polymer is selected from a
group consisting of a water-soluble polyacrylamide, a
carboxymethyl-cellulose, and poly (acrylic acid).
32. The solution of claim 29 wherein the solution also contains a
phosphate.
33. The solution of claim 29 wherein the solution also contains
phosphoric acid and another phosphate.
34. In a process for removing a selected portion of copper from a
printed circuit board having a substrate beneath the copper and a
mask covering another portion of the copper: contacting the board
with an aqueous solution of NO.sub.2 or HNO.sub.3, a sufficient
amount of a dissolved copper salt to prevent etching of the
substrate, a surfactant and an additive which inhibits undercutting
of the copper beneath the mask.
35. The process of claim 34 wherein the mask comprises a lead-tin
solder mask, and the aqueous solution also includes a
phosphate.
36. The process of claim 35 wherein the solution contains both
phosphoric acid and another phosphate.
Description
This invention pertains generally to the etching of metals, and
more particularly to a process for removing copper and other metals
in the manufacture of printed circuit boards.
Application Ser. No. 450,685, filed Dec. 17, 1982, describes a
process for etching patterns in laminated copper foils using a
gaseous nitrogen dioxide oxidant and an organic catalyst/solvent.
This approach dramatically simplifies the chemical etching step in
printed circuit board production compared to the wet etching
processes currently in use. It uses a simpler chemistry having
fewer process variables, it is less corrosive and thus permits the
use of standard materials for processing equipment, and it produces
less pollution and yields a single pure oxidized copper specie
which is readily disposed of.
However, notwithstanding these substantial advantages, the process
has been found to have certain limitations and disadvantages which
might limit its large scale development and commercialization. For
each mole of copper reacted, 2/3 mole of gaseous NO are produced,
and this causes considerable bubbling and foaming in the
catalyst/solvent layer which interferes with the reaction by
separating the reacting layer from the copper. This, in turn, leads
to non-uniform specific area reaction rates, since areas having
more exposed copper also have more vigorous gasing, thus reducing
the copper removal in these areas.
This system also has somewhat of a thermodynamic explosion
potential in that a rapid and uncontrolled reaction between
NO.sub.2 and the organic solvent mixture is energetically favored,
although the reaction of metals and NO.sub.2 in organic solvents
has been studied extensively without mishap.
In addition, the reaction of copper metal with NO.sub.2 is highly
exothermic, producing an excess of 80 kilocalories per mole of
copper reacted. The reaction is somewhat adiabatic, that is, the
heat of reaction is primarily absorbed by the system. Hence, the
temperature of the board increases rapidly during the reaction, and
this limits the thickness of the copper foil which can be removed.
Under conditions explored to date, the thickest foil which can be
removed by the adiabatic gas reaction appears to be about 1/2
oz./ft.sup.2, or a thickness of about 0.007" (18 microns). Attempts
to etch thicker foils produce disappointing results in that either
the reaction film goes dry giving an incomplete etch or the
substrate overheats, damaging the resist which destroys the
pattern.
It has now been found, somewhat surprisingly, that these problems
can be overcome and that even more improved results can be obtained
by utilizing water as a catalyst/solvent in the etching of copper
and other metals with nitrogen dioxide in the manufacture of
printed circuit boards. This discovery is surprising and unexpected
because NO.sub.2 reacts with water to produce nitric acid which
tends to attack both photoresist and substrate materials, two fatal
problems in the manufacture of circuit boards. However, it has been
found that by proper control of the process conditions, copper and
other metal can be removed rapidly from circuit boards without
damage to either the photoresist or the substrate, using an aqueous
solution of either NO.sub.2 or HNO.sub.3 as an oxidant.
It is in general an object of the invention to provide a new and
improved process for etching copper and other metals in the
manufacture of printed circuit boards and in other
applications.
Another object of the invention is to provide a process of the
above character which overcomes the limitations and disadvantages
of copper etching processes heretofore provided.
Another object of the invention is to provide a process of the
above character which is inexpensive and easy to carry out.
These and other objects are achieved in accordance with the
invention by utilizing water as a catalyst/solvent in the etching
of copper and other metals with nitrogen dioxide. In one disclosed
embodiment, a film of water is formed on the surface of the metal,
and the water-coated metal is exposed to gaseous NO.sub.2 to
dissolve the metal. In another disclosed embodiment, the metal is
exposed to an aqueous solution of NO.sub.2 or HNO.sub.3 and water,
either by immersion or by spraying to remove the metal. In either
embodiment, a polymer additive can be employed to prevent
undercutting or etching of the metal in a direction parallel to its
surface.
In the etching process of the invention, the oxidation of copper
takes place according to the following reaction: ##STR1## which
does not proceed in the absence of a suitable catalyst.
Since the reaction of NO.sub.2 with water superimposes the
relatively complex nitric acid forming chemistry upon the desired
copper etching chemistry, it may be helpful in understanding the
invention to consider some aspects of the nitric acid chemistry.
The overall process by which nitric acid is formed is given in the
following equation:
which is the sum of at least three independent steps represented by
the following equations:
It is important to note that all of these processes, including
reaction (2), are readily reversible. The formation of nitric acid
is favored by high NO.sub.2 pressures while it is retarded and even
destroyed by gaseous NO.
With gaseous NO.sub.2 and a copper foil circuit board (with resist
patterns), the invention is carried out by covering the board with
a thin film of water (e.g., 0.025" or less for a 1/2 oz./ft.sup.2
laminate) prior to exposure to the NO.sub.2. The circuit board is
exposed to the NO.sub.2 at room temperature, and the etching
process is found to begin almost immediately and to be
substantially complete within 2-3 minutes. At the end of the
reaction, the board is covered with a thin film of concentrated
copper nitrate which can be readily removed by a number of methods.
Since the Cu product is in aqueous solution with nitrate as the
only anion, the board can be made free of all residue by a simple
water wash because there are no Cu(I) species present. Since it is
difficult to form a uniform thin film with pure water, a polymer
additive with a surfactant co-promotor is employed to lower the
surface tension and increase the viscosity of the medium so that a
uniform thin film can be obtained. The additives are selected to
promote the anisotropic etching of copper foil so that the desired
pattern can be obtained with little or no undercutting of the
copper. Suitable polymers include water soluble polyacrylamides
such as the cationic Dow Chemical Separan CP-7HS and Hercules Reten
210, the neutral Hercules Reten 520 and the anionic Dow Chemical MG
700. Also included are water soluble poly(acrylic acids) such as
Aldrich Chemical #18, 127-7 and Rohm and Haas Acrysol A5 and
carboxymethyl-cellulose such as Hercules 12M31. Suitable
surfactants include DuPont surfactants Zonyl FSC, Zonyl FSN, Zonyl
FSP and Zonyl FSK, 3M Fluorad surfactant FC-135, Sherex, Adogen
477, and hexadecyltrimethyl-ammonium bromide.
The following examples demonstrate the use of a water film to
catalyze the etching of copper by gaseous NO.sub.2, and they also
show the influence of the polymer additive on the rate and course
of the reaction:
EXAMPLE 1
A 3".times.4" board with a laminated layer of 1/2 oz. copper per
ft.sup.2 having a test pattern of imaged Dynachem Laminar-ML dry
film photoresist was coated with 3.9 g of a 0.7% solution of Dow
Chemical Separan CP-7HS polymer in water. This composite was then
exposed to gaseous NO.sub.2 at 23.degree. C. for 2 minutes. After
an additional minute, the reaction mixture was washed from the
board with a simple water rinse. It was found that over 0.8 g of Cu
had been removed, essentially all of the exposed Cu in the test
pattern area, with virtually no undercutting of the pattern defined
by the photoresist.
EXAMPLE 2
The process of Example 1 was repeated using a test sample having an
etch pattern defined by an 4 micron thick layer of Kodak 752
Microresist. Under the same reaction conditions, over 0.8 of Cu was
removed in the area defined by the test pattern. The etching action
was such that lines having substantially vertical sides were
produced, although this sample was slightly undercut compared to
that in Example 1.
EXAMPLE 3
The process of Example 1 was repeated using a similar test panel.
This test sample was covered with 4.0 g of a 0.35% solution of
Separan CP-7HS in water. This sample was exposed to NO.sub.2 for
11/2 minutes, and the reaction layer was removed by a water rinse
after an additional 30 seconds. Again, 0.8 g of Cu was removed,
substantially all the exposed copper in the test pattern area, with
virtually no observed undercutting of the lines delineated by the
test pattern.
EXAMPLE 4
The process of Example 1 was repeated, using a similar test panel.
This sample was coated with 4.1 g layer of a 1% solution of
Hercules 12M31 carboxymethyl-cellulose in water. This sample was
exposed to NO.sub.2 for 2 minutes, and the reaction layer was
removed with a water rinse after an additional minute. It was found
that 0.3 g of Cu was removed from the test area, about 40% of the
exposed copper. While the etching of the pattern was incomplete,
there was virtually no undercutting of the lines protected by the
photoresist.
With nitrogen dioxide in an aqueous solution, the process looks
even more like a nitric acid etching process given the facile
equilibria relating H.sub.2 O, NO.sub.2 and HNO.sub.3, i.e.
equations (2)-(5). Nitric acid is a good oxidant and a strong acid,
and it is destructive of organic materials such as photoresist and
glass epoxy circuit boards. A system with good oxidizing power and
attenuated acidity, i.e. lower HNO.sub.3 concentration, should
oxidize the copper without destroying the organic components.
Fairly concentrated nitric acid is required for reaction with
copper because pure HNO.sub.3 in itself is unreactive toward
copper. For the reaction of equation (1) to occur, the nitric acid
must contain some dissolved nitrogen oxides. Hence, the reaction
probably proceeds through NO.sup.+ (or NO.sub.2.sup.+ in extremely
concentrated HNO.sub.3) arising from equations (3), (4) and (5).
From these equations, it can be seen that high acid (H.sup.+)
concentrations will promote high NO.sup.+ concentrations, driving
equation (4) backward. This indicates that a H.sub.2 O, NO.sub.2,
HNO.sub.3 system should be highly acidic in order to be reasonably
reactive toward copper. Unfortunately, such a system will also be
reactive toward organic materials.
It has been found that the NO.sup.+ concentration can be maximized
without high acidity by reducing the water concentration and its
chemical potential. This is achieved by using a copper salt such as
Cu(NO.sub.3).sub.2, the copper reaction product. Copper (II)
nitrate is extremely soluble in water; about 380 g of the salt
Cu(NO.sub.3).sub.2 .multidot.3H.sub.2 O will dissolve in 100 ml of
water at 40.degree. C. Any other salt of copper which is soluble in
HNO.sub.3 can likewise be used. Suitable salts include CuSO.sub.4,
copper (II) tetrafluoroborate, CuCl.sub.2 and combinations thereof.
With any of these salts, it is the copper ion Cu.sup.++ which
removes the water molecule from solution and makes it possible to
etch with HNO.sub.3.
By using concentrated solutions of Cu(NO.sub.3).sub.2 in water as
the reaction solvent, patterns can be rapidly etched in copper
laminate foils without damage to resists or substrates, using
either NO.sub.2 or HNO.sub.3 as the oxidant with both spray and
immersion techniques. The added NO.sub.2 (N.sub.2 O.sub.4) or
HNO.sub.3 is the sole source of oxidizing power in the system.
Hence, facile control of the etching parameters is readily
obtained. In contrast to other commercial wet etching processes,
the copper substrate is indifferent to the reaction medium
(concentrated aqueous Cu(NO.sub.3).sub.2) in the absence of added
oxidizer.
Due to the rapid interconversion between NO.sub.2 and HNO.sub.3 in
the system chemistry, it is possible to use solutions of HNO.sub.3
for copper oxidation. HNO.sub.3 is less expensive than pure
NO.sub.2 and is already in water solution, thus saving the expense
of removing the heat of reaction of NO.sub.2 with water. This water
is absorbed by the reacting copper and is retained in the hydrated
salt.
Use of this process for the etching of copper patterns in circuit
board foil is illustrated in the following examples:
EXAMPLE 5
A 3".times.4" board having a 1/2 oz./ft.sup.2 copper laminate with
a resist pattern formed from Kodak 752 Microresist was sprayed with
a mixture of 10 cc of 90% HNO.sub.3, and 50 cc of about 0.7%
solution of Separan CP-7HS in water, diluted with about 225 cc of a
40% by weight solution of Cu(NO.sub.3).sub.2 at
30.degree.-35.degree. C. The solution was recycled once. At the end
of this time substantially all of the copper was removed from the
pattern area with no undercutting of the resist pattern and no
damage to the resist or substrate.
EXAMPLE 6
A 3".times.4" board with a resist pattern formed with Dynachem
Laminar-ML film was sprayed with the solution of Example 1 after
aging for 24 hours and replenishing with 40 cc of 90% HNO.sub.3.
The board was sprayed with this solution at a rapid rate and was
substantially cleared of all Cu in the pattern area after 150 cc of
solution was used. Again, no resist or substrate damage was noted.
The pattern lines were very slightly undercut.
EXAMPLE 7
The process of Example 6 was repeated with an identical sample
except that the same reaction solution was diluted with 25 cc of a
1.4% solution of Separan CP-7HS in water. Again, the copper was
substantially removed from the pattern area after use of 150 cc of
solution. No resist or substrate damage was observed, and no
undercutting of the resist pattern was noted.
EXAMPLE 8
The solution of Example 7 was replenished with 5 cc of 90%
HNO.sub.3 and 20 cc of 1.4% Separan CP-7HS after the solution had
aged 24 hours. A 3".times.4" board having a resist pattern formed
from Dynachem Laminar-ML film was immersed in this solution with
agitation. The copper unprotected by the resist pattern was
completely removed in less than 2 minutes at 25.degree. C. There
was no resist or substrate damage evident, and there was no
noticeable undercutting of the resist pattern.
Similar results were obtained when NO.sub.2 was substituted for
HNO.sub.3 in the Cu(NO.sub.3).sub.2 solution.
This process is advantageous in that it requires very few chemical
components and, thus, affords relatively simple process control. It
is particularly suitable for use with thicker copper foils, i.e.
foils thicker than 1/2 oz./ft.sup.2. Corrosion problems are
minimized, and special materials such as titanium are not required
for the processing equipment. The chemistry is clean, and the
reaction product is extremely stable with no Cu(I) compounds to
generate an insoluble sludge. Copper is removed as pure
Cu(NO.sub.3).sub.2 0.3H.sub.2 O which has some market value itself.
If a mixture of Cu(NO.sub.3).sub.2 and CuSO.sub.4 is used as a
buffer, the CuSO.sub.4 0.5H.sub.2 O will precipitate first. The
process can be operated as a closed system, thus reducing pollution
problems. The process consumes only low cost nitric acid, which is
a readily available major commodity chemical, and it runs under
mild conditions, i.e. low temperatures. When the etching solution
becomes too concentrated in Cu(NO.sub.3).sub.2, it can be
precipitated from the solution by cooling the solution to around
10.degree. C. or by heating it to around 50.degree. C.
Other metals which can be etched by this process include vanadium,
manganese, iron, cobalt, nickel, palladium, and alloys of these
metals such as constantan and Monel. With each of these metals,
Cu(NO.sub.3).sub.2 can be employed as in the etching of copper to
maintain control of the reaction. Alternatively, a nitrate of the
metal being etched can be utilized instead of Cu(NO.sub.3).sub.2.
Thus, for example, Ni(NO.sub.3).sub.2 and Mn(NO.sub.3).sub.2 can be
used in the etching of nickel and manganese, respectively, and
Ni(NO.sub.3).sub.2 can also be used in the etching of nickel
alloys.
EXAMPLE 9--Nickel
A solution of 500 cc of 50% nickel nitrate (Ni(NO.sub.3).sub.2) 125
cc of 70% nitric acid (HNO.sub.3) and 20 cc of 7% Separan CP-7HS
(Dow Chemical polyacrylamide) heated to 45.degree. C. was used to
etch a 1".times.6" piece of nickel foil. About 0.37 g of Ni was
removed in 10 minutes; at 50.degree. C. 0.28 g of Ni was removed in
5 minutes.
EXAMPLE 10-Nickel
A solution of 4 liters of 50% Cu(NO.sub.3).sub.2 in water, 1.2
liters of 70% HNO.sub.3 and 200 cc of a 0.9% solution of Separan MG
700 (Dow Chemical poly(acrylamide)) was heated to 45.degree. C. and
used to etch a 1".times.6" piece of nickel foil. About 0.36 g of Ni
was removed in 1 minute.
EXAMPLE 11-Constantan
The solution, of Example 10 above, was used to etch a 41/2" long
piece of 20 gauge constantan (a Ni/Cu alloy) wire. About 0.068 g of
material was etched in 1 minute, an additional 2 minutes of
reaction removed another 0.137 g of material.
In etching copper circuit boards protected with a lead-tin solder
resist, appreciable chemical reaction of the resist can be
prevented by the addition of a small amount of phosphoric acid or
any other phosphate to the etching solutions disclosed herein, e.g.
nitric acid and copper nitrate with a polymer and surfactant. With
these additives, there is virtually no undercutting of the solder
etch mask, and this offers a significant improvement over existing
etch processes used for the production of plated-copper circuit
boards. Moreover, it has been found that the addition of a
fluorocarbon phosphate such as Zonyl FSP detergent to the
phosphoric acid makes the solder even less reactive, and it is
believed that the surface of the resist becomes covered with a lead
phosphate.
EXAMPLE 12
A solution of 3 liters of Cu(NO.sub.3).sub.2 in water, specific
gravity 1.50 at 20.degree. C., 1 liter of 70% HNO.sub.3, 500 cc of
85% H.sub.3 PO.sub.4, 15 cc of 3M Fluorad FC-135 (a cationic
fluorocarbon surfactant), 10 cc of DuPont Zonyl FSP (a fluorocarbon
phosphate), and 150 cc of a 1.1% solution of Reten 520 (a Hercules
polyacrylamide) was heated to 40.degree. C. and used to etch a
4".times.6" panel of copper laminate having a solder etch resist
pattern. The 1.4 mil layer of copper not covered by the solder
pattern was removed in 3 minutes. Examination of a cross-section of
this pattern showed that the copper was removed without noticeable
undercutting of the solder resist pattern.
It is apparent from the foregoing that a new and improved process
for etching copper has been provided. While only certain presently
preferred embodiments have been described in detail, as will be
apparent to those familiar with the art, certain changes and
modifications can be made without departing from the scope of the
invention as defined by the following claims.
* * * * *